An optical scanning device (an exposure device) mounted in an electrophotographic image forming apparatus includes a light source for emitting a light beam, a scanning optical system for deflecting and scanning a surface to be scanned by the light beam from the light source, and the like. Furthermore, each member constituting the optical scanning device, such as the light source and the scanning optical system, is held to a casing of the optical scanning device. In addition, the scanning optical system is received in a receiving area provided in the casing in order to suppress dust such as scattered toner from being attached to the scanning optical system.
Furthermore, in the optical scanning device, a detection member for detecting the light beam emitted from the light source is installed (for example, see Patent Literature 1). The detection member includes a light receiving element such as a photodiode, receives and detects the light beam, and outputs a detection signal indicating that the light beam have been detected. Furthermore, a control unit for controlling an operation of the optical scanning device measures a start timing of optical scanning on the basis of the detection signal from the detection member.